Hydraulic fracturing is a common method for stimulating the production of hydrocarbon bearing formations. Fracturing includes the steps of injecting a fluid down a wellbore at high pressure to “crack’ the rock and create openings for the hydrocarbons to flow back to the well. The fracturing fluid may contain proppant such as sand or gravel to hold open the created openings in the formation. The fracturing fluid typically contains large amounts of fluid, such as water. This water is called source water and is generally potable. The fracturing fluid will also contain chemicals to aid in proppant transport, friction reduction, wettability, pH control and bacterial control. The fluid is mixed with the chemicals and proppant, and pumped down a wellbore at high pressure and into the cracks in the formation.
When using proppant, it is often necessary to include a thickener in the fluid to carry the proppant to the formation and into the cracks. Thickeners such as guar as often used since they will thin under shear during pumping but thicken at the formation when shear is no longer present. The guar may also cross link in the fluid to increase the fluid's ability to effectively carry proppant.
The standard method for producing a crosslinked hydrating guar fracturing fluid is to add a guar to source water and allow the guar to hydrate in the water; towards the end of the hydration, if necessary adjust the pH with a potassium or sodium hydroxide base; and add a crosslinker to the water. In some cases, the addition of the base and crosslinker may occur at the same time.
When producing hydrocarbons from the formation, water may also be produced including formation water, injected water from secondary recovery operations (for example waterflooding), and flowback water from completion and remedial operations. Produced water often includes other components including for example petroleum hydrocarbons, suspended and dissolved solids, acid-carbonate reaction by-products from perforation and acidizing operations, spent fracturing fluid additives, and residual production chemicals. Produced water may include water existing in formations such as aquifer water, or brackish or brine water.
Due to the large amounts of water that are required in a typical fracturing operation, it is economical and environmentally preferable to recycle produced water from a well or nearby wells and use it in fracturing fluids.
Produced water often has a high concentration of total dissolved solids (TDS), many of which are known to interfere with the stability and/or compatibility of hydraulic fracturing fluids. In particular, crosslinking guars used in hydraulic fracturing fluids typically are limited by the concentration of total dissolved solids or salts, the concentration of monovalent ions such as not but limited to sodium, potassium, bromide, bicarbonate, and chloride, and by the concentration of multivalent ions such as but not limited to calcium, magnesium, manganese, barium, strontium, iron, sulfate or carbonate. Each of these negatively influences gel hydration rates, gel cross-linking efficiency, shear stability, and gel thermal stability. As a result, the base fluid used in cross-linked hydraulic fracturing fluids has been limited and potable water is commonly used.
Potable water would typically contain less than 3000 mg/L of TDS. Brackish water would typically contain about 3,000-10,000 mg/L of TDS. Saline water would contain over 10,000 mg/L of TDS while brines having a salinity greater than seawater typically have about 35,000 mg/L. Water suitable for use in fracturing fluids generally contain less than about 42,000 mg/L of TDS (although lower amounts are preferred) to limit the interference of these components with the formation and operation of the fracturing fluid. Some problems in forming fracturing fluids from water having high concentrations of TDS may include interference with the hydration of the polymer, formation of scale, interference with the breakers, polymer oxidation, unwanted delay of crosslinking, and crosslinker precipitation.
Typical targets for use inWater Propertyhydraulic fracturing fluidspH6-8.5Calcium and Magnesium<2000ppmIron<10ppmBarium and Strontium<5ppmChlorides<40,000ppmBicarbonates<300ppmPhosphates<5ppmSulphates<500ppm
Boron present in produced water may be problematic in recycling the produced water for fracturing applications. In some crosslinking guar systems, boron is used in the crosslinking process. The boron in solution in the produced water may participate in crosslinking the polymer solution formed in the produced water and compete with the boron purposefully added as the crosslinker. The additional boron in the produced fluid may cause the fluid to crosslink prematurely, and/or overcrosslink. Boron levels in produced fluid may be 10 mg/L or higher but typically operators look for water with boron levels less than this level for use in hydraulic fracturing fluids since this is generally considered to be low enough not to interfere with the crosslinking process.
It is therefore desirable to be able to avoid the use of potable water in fracturing fluids and instead use produced water from formations in fracturing formations without negatively affecting the fracturing fluid formation or operation including the cross-linking gel hydration in the fluid. The use of produced water would provide economic as well as environmental benefits to operators.